Conventional mobile communications systems, particularly cellular systems, contain a number of base stations which communicate with mobile stations. If at least two base stations communicate simultaneously with the same mobile station, so-called macrodiversity transmission takes place. This mode of transmission is used, for example, in code division multiple access (CDMA) mobile communications systems. In the planned mobile communications system UMTS (universal mobile telecommunications system), macrodiversity transmission is to be used at least during handoff of the mobile station from one base station to another. In the UMTS, the powers transmitted by the base stations must be adapted as quickly as possible to changes in channel characteristics, which are caused by fading, for example. The mobile station sends to all base stations serving it a transmit power control (TPC) command instructing them to change their transmitted powers, namely to raise or lower the transmitted power level by a predetermined amount of, e.g., 1 dB. This process thus represents a closed-loop power control mechanism.
The invention is predicated on recognition that the following problem arises if TPC commands are detected in error. If one of the base stations receives the TPC command incorrectly, it will change its transmitted power incorrectly as compared with those base stations which receive the TPC command correctly.
This means that this individual base station will change its transmitted power in the opposite sense to the other base stations. Accordingly, the transmitted power levels will drift apart (xe2x80x9crandom walkxe2x80x9d). This drift reduces the desired macrodiversity gain. To prevent this drift, according to the invention, information about the powers currently being transmitted by the base stations is transmitted at predeterminable time intervals to a base station controller which evaluates this information to calculate a power correction value for at least one of the base stations, and the base station controller transmits the correction value to the base station, which then additionally changes its transmitted power by this correction value. Accordingly, the transmitted power levels are monitored by the base station controller at regular time intervals in order to determine whether the transmitted power levels are drifting apart, and to perform a transmit power correction in at least one of the base stations if necessary. Thus, in addition to executing the known change in transmitted power in response to the TPC command, the base station to which a correction value is communicated will correct this change if a drift of the transmitted power levels is detected.
This method, in which the decision of the base station controller to instruct a base station to correct its transmitted power is made based on feedback information (transmit level data) about the actual transmitted powers of the base stations, will be referred to herein as xe2x80x9cfeedback-type transmit power correctionxe2x80x9d. One advantage of the proposed method is that the transmitted power levels cannot drift very far apart, because the transmitted power is corrected right at the beginning of a drift.
Further advantageous aspects of the invention are defined in the subclaims.